On the combination of the planktonic foraminiferal Mg/Ca, clumped (Δ₄₇) and conventional (δ¹⁸O) stable isotope paleothermometers in palaeoceanographic studies

Assuming that foraminiferal clumped isotope (Δ₄₇) values are independent of seawater salinity and pH, the combination of Mg/Ca, δ¹⁸O and Δ₄₇ values, may in theory allow us to disentangle the temperature, salinity/δ¹⁸Osw and pH signals. Here, we present a new Mg/Ca-Δ₄₇ dataset for modern planktonic foraminifera, from various oceanographic basins and covering a large range of temperatures (from 0.2 to 25.4 °C). These measurements were performed on the same samples and species as the ones used for the foraminiferal Δ₄₇ calibration of Peral et al. (2018), allowing comparison between both Mg/Ca and Δ₄₇ paleothermometers (excluding the two benthic foraminiferal data points). There is a good agreement between these two paleothermometers when the Mg/Ca-temperature is corrected for seawater salinity and pH, suggesting that foraminiferal Δ₄₇ may not be influenced by salinity or pH. However, our results show that Δ₄₇ temperature uncertainties still limit our ability to reconstruct pH and δ¹⁸O_sw from the combination of Mg/Ca, δ¹⁸O and Δ₄₇ in a useful manner. We also find that disagreements between Mg/Ca and Δ₄₇ values in G. bulloides persist after correction for vital, salinity and pH effects, suggesting that other process(es) may also influence Mg/Ca in this species. This study also provides an updated I-CDES version of the previously published planktonic and benthic foraminiferal Δ₄₇ calibration of Peral et al. (2018), covering a range of temperature from −2 to 25.4 °C.